This invention relates to a process for the improvement of the physical and chemical properties of mixtures of polyvinyl chloride and finely divided synthetic silicic acid. More particularly, it relates to the use of hexanetriol with such mixtures to obtain finished products having increased hardness, heat stability and resistance to discoloration.
PVC is one of the most widely employed plastic materials. The thermal plasticity of this material frequently limits its application, especially when used for shoe soles, profiles, and hoses made of hard or soft PVC, as well as floor coverings. The thermal plasticity at low temperatures can be improved by the use of plasticizers (e.g., phthalates, sebacates, etc.); however, use of plasticizers reduces the heat stability of the plastic, causing stickness, a greasy grip or a greasy surface sheen. It is known in the art that these latter characteristics can be overcome by the addition of small amounts (e.g., 1 to 3%) of a pyrogenic or precipitated silicic acid. The addition of larger amounts of the silicic acid would be desirable. This is especially true with the relatively low cost precipitated silicic acids. However, larger additions generally result in an undesirable darkening of the finished product, probably as a result of increased HCl liberation. Consequently, the large quantities of silicic acid required to obtain improved heat stability cannot generally be enployed.
Although the use of natural fillers, particularly chalks and kaolins, is known in the art, these fillers do not bring about any improvement in the physical and chemical properties of PVC mixtures. Rather, they merely serve to reduce the cost of finished products having less stringent application requirements.
Consequently, there exists a need for a process for improving the physical and chemical properties of mixtures of polyvinyl chloride (PVC) with finely divided synthetic silicic acids, whereby finished products are obtained which exhibit markedly greater hardness and heat stability, and which at the same time are free from discoloration. The finished products should be particularly suited for soles of shoes and floor coverings.